The present invention relates generally to nuclear fuel elements comprising a cladding tube and a plurality of cylindrical fuel pellets stacked within the cladding tube. It relates, more particularly, to fuel elements in which stress at the inner surface of a cladding tube does not concentrate in a limited portion of the tube.
When a plurality of cylindrical fuel pellets are inserted and stacked in the cladding tube to form a nuclear fuel element, there is generally provided a gap of about 0.15 mm to 0.30 mm in the diameter between the cladding tube and the pellets disposed therein. The value of the clearance has been adjusted by grinding the outer surface of the pellets after they are sintered in the larger dimensional shape than nominal one, because the pellets are likely to form an hour-glass shape in which the upper and lower portions exhibit larger diameter than the middle portion.
When, however, the surface ground fuel element is exposed to the high temperature and radiation environments in a nuclear reactor, the fuel pellets exhibit a number of cracks therein due to a large temperature difference between the outer circumferential region and the central region of the pellets, with a result of forming a number of fragments of the pellets. FIG. 1 shows the thus cracked pellet 2 encapsuled in the cladding tube 1. The fragments produced by the cracks are located in a close abutment with the inner surface of the cladding tube. The fragments then produce a local stress on the cladding tube due to a thermal expansion and swelling etc, during the increase of the power output of the nuclear reactor. A further increase of the output urges the pellets to be deformed to form such a shape as schematically shown in FIG. 2, in which the cylindrical external lateral surface of the pellet is warped outwardly from the central exis of the pellet, especially for the both ends of the pellet. The outward warping is due to a small difference of fuel density in the longitudinal direction of the pellet and due to the temperature difference between the central and outer regions of the pellet. Thus, as shown in FIG. 3, stress at the inner surface of the cladding tube 1 facing to a crack opening 7 is largely developed in a limited section of the cladding tube to cause concentration of strain. If such strain concentration is caused, the cladding tube is broken at the strain-concentrated portion, even through the total strain caused in the circumferential direction of the cladding tube 1 is relatively small.